What is an evaporation line?
An evaporation line, also called an evap line, is a faint line that can appear on a pregnancy test as the urine sample dries. As urine dries on a pregnancy test strip, the chemicals that detect hCG (human chorionic gonadotropin, the pregnancy hormone) can get concentrated and cause a faint vertical line to appear. This line often shows up after the recommended testing time window and is caused by the natural drying of urine rather than being an accurate test result.
How does an evaporation line occur?
Pregnancy tests work by wicking urine via capillary action along the test strip to an absorbent pad. The test strip contains antibodies that bind to hCG if present in the urine. This triggers a color change reaction in the test line region, typically producing a pink or purple line.
As the urine sample begins to dry and evaporate on the test strip, the urine can concentrate and accumulate at the edges. This may deposit higher amounts of hCG at the test line location. Even very low or trace amounts of hCG that are not high enough to indicate pregnancy can then become visible as a thin line or faint coloring on the test area.
However, an evap line does not mean the test is positive. It is an artifact of the drying urine rather than a true reflection of hCG levels.
How common are evaporation lines?
Evaporation lines are quite common on pregnancy tests. One study found that evaporation lines occurred in over 60% of urine samples that were allowed to dry on a pregnancy test past the manufacturer’s recommended testing time.
The likelihood of getting an evap line increases if:
- The test is read after the instructed time (e.g. after 10 minutes).
- The air is very dry, which accelerates evaporation.
- The urine sample is very concentrated, having sat for a while before testing.
- You are taking fertility medications containing hCG.
- The pregnancy test is less sensitive and uses a higher hCG cutoff level.
- The test has been taken apart or damaged.
Proper testing technique can reduce evaporation lines. This includes:
- Reading the results within the specified testing time window.
- Not removing the test stick from urine mid-stream.
- Discarding the test promptly after a negative result.
What does an evaporation line look like?
Evaporation lines look like thin, faint lines that appear outside the time limit on a pregnancy test. They are usually colorless, gray or light pink. Evap lines tend to appear uniform in color and thickness, while true positive pregnancy test lines are normally thicker and vividly colored.
Some key characteristics of evaporation lines:
- Faint, thin and colorless/pale in color
- Appears outside the manufacturer’s testing time window
- Usually shows up after 10 minutes or more
- May slowly develop hours after taking the test
- Is vertical and uniform in thickness
- Can be jagged or dotted rather than a solid line
Are evaporation lines always colorless?
Most evaporation lines have no significant color to them. However, it is possible in some cases to get a faintly colored evap line, especially on more sensitive pregnancy tests.
This happens when trace amounts of hCG in the urine become concentrated as drying occurs. The small quantities of hCG may be enough to trigger a very faint color reaction at the results line. So you might end up with a barely pink or purplish tinted evap line in some instances.
Can evap lines have color when not pregnant?
Yes, it is possible to get a light colored evap line even when you are not pregnant. Some potential reasons for this include:
- Highly sensitive pregnancy test detecting trace hCG
- Residual hCG left over from previous pregnancy
- Recent miscarriage or abortion causing hCG decline
- Ectopic or molar pregnancy producing low hCG
- Pituitary disorders leading to elevated hCG
- Hormone medications like fertility treatments
If a colored line appears within the time limit, even if faint, it should be considered potentially positive. Re-test with a first morning urine sample to confirm.
Can an evap line be a false positive?
Evaporation lines do not indicate pregnancy. The appearance of a typical thin evap line outside the testing time window does not mean the test is positive.
However, if you see any pink or colored line appear within the specified testing time on a pregnancy test, even if faint, this could potentially signify pregnancy. It should not be completely ignored as just an evaporation line. Re-test first thing the next morning to be sure.
How long after can evap lines form?
Evaporation lines usually begin to appear after the recommended testing time, from 5 minutes onwards. However, it can sometimes take hours for an evap line to develop fully. This depends on the test and conditions like temperature and humidity.
In some cases, evap lines may not show up until 1-2 days after taking the test. Leaving tests lying around can allow eventual evap lines as the urine sample fully dries.
Typical evaporation line timeframes:
- 5-10 minutes – common time for faint lines to start forming
- 10-30 minutes – evap lines begin to become more visible
- 1-2 hours – evap lines darken and stabilize
- 12-48 hours – very dry tests can develop dark evap lines
The key is that if an evap appears after the manufacturer’s recommended result reading time, it should not be considered a positive test.
Can evap lines get darker over time?
Yes, evaporation lines often start out faint when they first appear after the reaction time, but then may become darker and more prominent as the urine dries. How dark an evap line gets depends on:
- The properties of the specific test
- Environmental temperature and humidity
- How long the test is allowed to dry
- How concentrated the urine sample was
In some cases, evap lines can end up fairly dark and prominent if the test fully dries over a longer period. But the darkness does not correlate to being pregnant. Only lines that appear within the manufacturer’s instructions should be read.
How long do evap lines last?
An evaporation line will last as long as the test strip remains absorbent and retains dried urine. Typically, evap lines begin fading after 1-2 days as the urine fully dries and dissipates. But they may last up to several weeks if the test is stored in cool, humid conditions that slow evaporation.
Moisture can also make dried tests reabsorb traces of the evap line. Once the test strip is fully dry, any remaining evap line will permanently disappear.
Can evap lines on pregnancy tests be prevented?
It can be difficult to completely prevent evaporation lines, since they are an inherent issue as urine samples dry. However, you can reduce the chance of getting a misleading evap line by:
- Following the test instructions precisely.
- Reading results within the specified time limit.
- Not removing test stick mid-stream when collecting urine.
- Testing first morning urine which is more concentrated.
- Storing tests properly before use.
- Discarding tests promptly once negative results are read.
Using a high quality test from a reputable brand can also help minimize evaporation line problems.
What do you do if you see an evaporation line?
If a faint or ambiguous line appears outside the manufacturer’s recommended testing time window, it should be considered an evaporation line. What you should do:
- Note the time the line appeared versus test time limit.
- Check if line has any color and compare to control line.
- Do not interpret as positive result if appearing after time window.
- Retest with a new pregnancy test using first morning urine.
- If line appears within time on re-test, consider it positive.
See your doctor right away if your period is late and you are still getting ambiguous results. A blood hCG test can provide definitive confirmation of pregnancy status.
Conclusion
Evaporation lines are a common occurrence with pregnancy tests, but can cause confusion around proper test interpretation. Any faint or ambiguous line that shows up after the manufacturer’s recommended testing time should not be considered positive, even if slightly colored. Always re-test using concentrated first morning urine if you see a late-appearing evap line to confirm your results. While evap lines are unavoidable as urine dries, following proper testing technique can help minimize misleading or confusing evaporation lines.